limited private types, "=" operator

[erickson@cs.nps.navy.mil (David Erickson)]

Is there any way to define "=" for a limited private type which is an
access type (which may be null)?  The problem arises trying to test for
null without producing an infinitely recursive definition.

The only solution that I am aware of is to use UNCHECKED_CONVERSION to
retype the access type during the test for null, but this is a hack.
Is there a cleaner solution?

Thanks -

	Dave Erickson